It comes after soldiers died in an SAS training exercise in the Brecon Beacons in 2013, also on one of the hottest days of the year.

The London Ambulance Service said it had had 300 more calls than usual and the RSPCA said it had also seen a spike in calls from members of the public concerned about dogs left in hot cars, conservatories, sheds and caravans.

Dr Angie Bone, head of extreme events at Public Health England, said: “Older people, those with underlying health conditions and young children may all feel the ill-effects of heat over the coming days.”

The NHS specifically advises that babies, children or elderly people are not left alone in stationary cars. Further information on how to keep cool during the heatwave can be found here.